NiO nanoparticles with plate structure grown on graphene as fast charge–discharge anode material for lithium ion batteries

Abstract

A graphene/NiO composite was prepared by simple chemical precipitation followed by thermal annealing. The results of XRD, FT-IR, FE-SEM, FE-TEM, and EDS analyses confirmed the presence of NiO nanoparticles with plate structure on the graphene surface. The discharge capacities of graphene, graphene/NiO (37 wt.%), and graphene/NiO (59 wt.%) are about 302, 604, and 856 mAh g⁻¹ at 5000 mA g⁻¹ (5 C), respectively. The cells containing 59 wt.% NiO show the best performance, and the graphene nanocomposite materials have high rate properties that are comparable to some of the best results reported in the literature using NiO. This graphene/NiO (37 and 59 wt.%) nanocomposite displays superior LIB performance with large reversible capacity, high Coulombic efficiency, good cyclic performance, and excellent rate capability, highlighting the importance of the anchoring of nanoplate structure NiO on graphene sheets for maximum utilization of electrochemically active graphene/NiO for energy storage applications in high-performance LIB.

Introduction

Lithium-ion batteries (LIB), as power sources for mobile communication devices, portable electronic devices, and electrical/hybrid vehicles, have attracted special attention in the scientific and industrial fields due to their high electromotive force and high energy density. For the anode material in LIB, graphite is usually employed as a standard electrode because it can be reversibly charged and discharged under intercalation potentials with reasonable specific capacity [1], [2]. However, to meet the increasing demand for batteries with higher energy density, much research has explored new electrode materials and designed novel nanostructures of electrode materials [3]. In the past decade, many new anode materials for lithium-ion batteries have been explored, with the aim to improve the capacity and energy density of the battery system. Those anode materials include: (i) nanosize alloys and intermetallic alloys. Elements such as Sn, Si, and Sb, etc. can reversibly react with lithium, in order to form lithium metal alloys [4], [5]. However, these alloying reactions are always accompanied by large volume changes in the electrode, inducing electrode failure. (ii) Transition metal phosphides and lithium nitrides using displacement reactions [6]. (iii) Transition metal oxides. Many binary transition metal oxides, such as Co₃O₄, Fe₂O₃, FeO, Cu₂O, CuO, and NiO, can be used for reversible lithium storage based on the conversion reaction. CoO (∼700 mAh g⁻¹) and Co₃O₄ (∼1000 mAh g⁻¹) discharge to lower potentials (0.01 V), but a significant drop in capacity was observed after the first discharge; thereafter, cells cycled well with ∼100% capacity retention to 100 cycles. Other metal oxides, such as Cu₂O, FeO, and NiO, show similar behavior but with poorer cycling performance [7], [8], [9].

Graphene, a monolayer of carbon (i.e. carbon atoms in a two-dimensional (2D) honeycomb lattice), has been found to exist in free-standing form and it exhibits many intriguing physical properties [10], [11]. Due to the high quality of the sp² carbon lattice, electrons were found to move ballistically in the graphene layer even at ambient temperature [12], [13]. Also, graphene has an ideal single-atom thick substrate for growth of functional nanomaterials, to render them electrochemically active and electrically conductive to outside current collectors. During the intercalation process, lithium transfers its 2s electrons to the carbon host and is situated between the carbon sheets. High capacity carbon materials have also been reported. Their high capacity can mainly be ascribed to (i) lithium insertion within the “cavities” in carbon [14], (ii) lithium absorption on both sides of the carbon sheet [15], and (iii) lithium binding on the so-called “covalent” sites [16]. Owing to its large surface-to-volume ratio and highly conductive nature, graphene may deliver a high lithium storage capacity in lithium-ion batteries. Recently, the Li ion storage capacity of graphene was found to be 784 mAh g⁻¹ [17], which is twice the capacity of graphite. The reason may be that graphene adsorbs Li ions on its two equal surfaces, forming a LiC₃ compound.

Reports on graphene composites with Si [18], TiO₂ [19], Co₃O₄ [20], Fe₃O₄ [21], SnO₂ [22], [23], [24], Cu₂O [25], and CuO [26] claim that a uniform distribution of metal oxide on graphene sheets can eliminate restacking of the sheets during the synthesis and that it stabilizes the volume changes in the metal/metal oxide during charge–discharge cycling. However, the performance at high current densities such as 100–5000 mA g⁻¹ has not been investigated. Quite recently, graphene–nickel oxide nanostructures have been synthesized using a controlled hydrothermal method, which enabled in situ formation of NiO with a coralloid nanostructure on graphene. Graphene/NiO (20 wt.%) and graphene/NiO (50 wt.%) show stable discharge capacities of 185 mAh g⁻¹ at 6000 mA g⁻¹ and 450 mAh g⁻¹ at 300 mA g⁻¹ [27].

In this paper, we report a simple strategy for synthesizing such NiO composites anchored on conducting graphene as an advanced anode material for high performance LIB. The resulting NiO nanoparticles with plate structure are 90–120 nm and homogeneously anchor on graphene sheets as spacers to keep the neighboring sheets separated. The graphene sheets overlap with each other to afford a three-dimensional conducting network for fast electron transfer between the active materials and the charge collector. This graphene/NiO (37, 59 wt.%) nanocomposite exhibits superior LIB performance with large reversible capacity, high Coulombic efficiency, good cyclic performance, and excellent rate capability, highlighting the importance of the anchoring of nanoplate structure NiO on graphene sheets for maximum utilization of electrochemically active grapheme/NiO for energy storage applications in high-performance LIB.